1. Field of the Invention
The present invention relates to a suspension device for an automotive vehicle and, more particularly, to a hydraulically adjustable suspension device which is suitable for orientation or attitude control as well as leveling control of the automotive vehicle.
2. Description of Related Art
In order to maintain a nearly constant curb height or road clearance, i.e., distance from a bumper, frame, etc. to the ground, regardless of load changes on the vehicle body, various kinds of level adjustable suspension systems have been made commercially available.
For the purpose of providing a brief background that will enhance an understanding of the present invention, reference is made to FIG. 7, which is an illustration of a practical structure of a known hydraulically adjustable suspension system.
In FIG. 7, a pair of, for instance, rear hydraulic suspensions 10 includes respective hydraulic cylinders 11. Each hydraulic cylinder 11 includes a cylindrical tube 11T and a piston rod 11P which reciprocally slides in the cylindrical tube 11T. The piston rod 11P is provided with an accumulator 12 for assisting the piston rod 11P in moving up and down quickly, yet smoothly, through a damping force generating mechanism, such as a pressure reducing valve 13, which removes or reduces oscillation of hydraulic pressure circulated between the hydraulic cylinder 11 and the accumulator 12. The piston rod 11P protrudes from and is retracted into the cylindrical tube 11T. The hydraulic suspensions 10 are operated by a suspension control hydraulic (SCH) circuit, including a hydraulic oil pump 1 driven by an engine (not shown).
Hydraulic oil pump 1 draws out oil contained in an oil tank 2 through an oil intake pipe 3 and discharges and delivers pressurized oil through an oil feed pipe 4. The feed oil pipe 4 is provided with a valve and an accumulator, in that order from the oil pump 1. More particularly, in the system shown in FIG. 7, the oil feed pipe 4 is provided with a check valve 5 for preventing back flow of the discharged oil toward the oil pump 1 and an accumulator 6 for reserving the pressurized oil. The oil feed pipe 4 branches off into four branch oil feed pipes 4A (two of which are shown) for the front and rear hydraulic suspensions 10. A flow rate control means, such as a servo control valve 7, which is a proportional type of solenoid operated hydraulic servo valve, is connected to each branch oil feed pipe 4A. The servo control valve 7, which may be of any well known type, has a spool (not shown) which is operated by a solenoid (not shown) so as to occupy three operative positions. These positions are an oil feed position (A), in which the servo control valve 7 allows oil to be applied to the cylinder 10 for expansion, an oil discharge position (B), in which the servo control valve 7 allows oil to be discharged from the cylinder 10 for retraction, and a normal position (C), between the oil feed position (A) and the oil discharge position (B), in which the cylinder 10 is kept neutral. With a change in intensity of solenoid current, the servo control valve 7 causes the spool to be continuously displaced so as to vary the valve opening linearly. Generally, because of the spool, each servo control valve 7, when in the normal position (C), may allow oil leakage in the branch oil feed pipe 4A, a feed-return oil pipe 8, and a oil return pipe 9.
Feed-return oil pipe 8 is connected between the servo control valve 7 and the suspension 10. The oil return pipe 9 is connected between the servo control valve 7 and the oil tank 2. Specifically, the oil return pipes 9 are joined together so as to form a common return pipe portion 9A, through which oil is returned into the oil tank 2. In each feed-return oil pipe 8, there is provided a pilot pressure check valve 14 for preventing oil leakage from the cylinder 11 through the servo control valve 7 when it is in the normal position (C) during, for example, long periods of engine stoppage. If there is actually oil leakage, the road clearance of the vehicle will be decreased by at least the weight of the vehicle. The pilot pressure check valve 14 functions as a normal check valve so as to prevent back flow of the discharged oil from the cylinder 11 toward the oil tank 2 through the feed-return oil pipe 8 before it is subjected to high pilot pressure oil passed through a pilot pressure control valve 16, which will be described later. On the other hand, the pilot pressure check valve 14 allows a flow of oil in both directions in the feed-return oil pipe 8 while it is subjected to high pilot pressure oil passed through the pilot pressure control valve 16.
Pilot pressure control valve 16, which may be, for instance, a three port, two position electric solenoid operated directional valve, is connected to the oil feed pipe 4 at a juncture with the branch oil feed pipes 4A and the oil return pipe 9 through connecting oil pipes 17 and 18, respectively. The pilot pressure control valve 16 is connected to both of the pilot pressure check valves 14 through a connecting oil pipe 15A branching off into two pilot pressure oil pipes 15. One of the two pilot pressure oil pipes 15 is connected to each of the pilot pressure check valves 14. The pilot pressure control valve 16, thus arranged, selectively connects and disconnects the oil flow between the oil feed pipe 4 and the oil return pipe 9.
Oil feed pipe 4, upstream of the check valve 5, and the common return pipe portion 9A of the oil return pipe 9 are communicated with each other by means of an oil unload pipe 19 with a stop valve 20. The stop valve 20, which is a two port, two position electric solenoid operated valve, occupies two positions, i.e., a load position (D) and an unload position (E). The stop valve 20 is normally forced to occupy the load position (D) so as to close the unload oil pipe 19, thereby loading pressurized oil into the accumulator 6. However, when pressurized oil in the oil feed pipe 4 reaches a predetermined active pressure, which is detected by a pressure sensor, the stop valve 20 is forced to the unload position (D) and opens the oil unload pipe 19, thereby allowing pressurized oil from the oil pump 1 to circulate into the oil tank 2 through the opened oil unload pipe 19. The pilot pressure control valve 16 may be adapted to ordinarily communicate the oil return pipe 9 and the pilot pressure oil pipe 15 with each other. In such a case, the pilot pressure control valve 16 is electrically caused, simultaneously with the change in position of the servo control valve 7 to the oil discharge position (B), to communicate the oil feed pipe 4 and the pilot pressure oil pipe 15 with each other so as to open the pilot pressure check valves 14. Alternatively, the pilot pressure control valve 16 may be electrically caused, upon starting of the engine, to communicate the oil feed pipe 4 and the pilot pressure oil pipe 15 with each other so as to open the pilot pressure check valves 14.
The solenoid operated valves 7, 16 and 20 are electrically controlled by a control unit (not shown) according to vehicle conditions, such as a total vehicle loads and vehicle speeds, so as to provide the most desirable road clearance.
In a known hydraulically adjustable suspension system constructed in this way, once the oil pump 1 is actuated by the engine, the pressurized oil discharged from the oil pump 1 accumulates or is loaded in the accumulator 6 through the check valve 5. Then, when the pressurized oil is detected to have reached the predetermined active pressure, the stop valve 20 is forced to occupy the unload position (E), so as to open the oil unload pipe 19, thereby allowing pressurized oil from the oil pump 1 to circulate into the oil tank 2 through the opened oil unload pipe 19. As a result, pressurized oil in the oil feed pipe 4 is maintained at the predetermined active pressure. If a road clearance of the vehicle decreases due to increased vehicle loads, a level sensor detects the decrease in road clearance and causes the suspension control hydraulic (SCH) circuit to shift servo control valves 7 to the oil feed position (A) from the normal position (C). This allows the pressurized oil in the accumulator 6 to be fed to the hydraulic cylinders 11 of the suspensions 10 through the pilot pressure check valves 14, respectively, so as to expand the hydraulic cylinders 11 and elevate the vehicle, thereby maintaining a given road clearance of the vehicle. If, however, the road clearance of the vehicle increases due to decreased vehicle loads, the suspension control hydraulic (SCH) circuit shifts servo control valves 7 to the oil discharge position (B) from the normal position (C), allowing pressurized oil in the hydraulic cylinders 11 of the suspensions 10 to flow to the oil tank 2 through the feed-return oil pipes 8 and the oil return pipes 9 via respective pilot pressure check valves 14. As a result, the hydraulic cylinders 11 contract and lower down the vehicle, so as to maintain a given clearance between the road and the vehicle.
While maintaining a given clearance between the road and the vehicle, the piston rods 11P of the cylinders 10 may possibly undergo positional changes relative to their respective cylinder tubes 11T which are different from one another, due to inertial forces generated during steering. This may result in an inclined vehicle attitude. If in fact there are differences in suspension levels among the four suspensions 10, level sensors detect the levels of the suspensions 10 so that the respective servo control valves 7 can be properly managed so as to be selectively shifted to their oil feed positions (A) and oil discharge positions (B). The openings of the servo control valves, therefore, can be managed according to the differences for vehicle attitude control.
From a layout standpoint, the servo control valve 7 must be arranged between the cylinder 11 and the oil pump 1 and oil tank 2, and between the oil feed pipe 4 and the feed-return oil pipe 8 and the oil return pipe 9. Also, the servo control valve 7 is most practically located between the oil pump 1, which is typically located near an engine, and one of the hydraulic cylinders 11, which must be directly incorporated in a suspension 10. Consequently, there is a problem in that the known hydraulically adjustable suspension system lacks readiness and space in which the servo control valves can be assembled.
Another problem with the known hydraulically adjustable suspension system is that the hydraulic cylinder 11 is somewhat sluggish in response to shifting of the servo control valve 7. This is because when the servo control valve 7 shifts to the oil feed position (A) from the normal position (C), oil passing through the servo control valve 7 takes an excessive time to reach the hydraulic cylinder 11 because of the length of the feed-return oil pipe 8 between the servo control valve 7 and the hydraulic cylinder 11. This results in a high resistance of the feed-return oil pipe 8 to oil flow. If the hydraulic cylinder 11 is sluggish in response, particularly while the vehicle travels at a high speed, controlling attitude of the vehicle stably is difficult, even though a vehicle attitude control is performed. This adversely affects the safety of the vehicle as it travels.